Aerosol charge conditioner

ABSTRACT

An attenuated soft x-ray neutralizer for neutralizing aerosols. The apparatus includes a soft x-ray emitter that emits soft x-rays into an aerosol conditioning chamber. An attenuating window may be included that reduces the intensity of the soft x-rays that bombard the aerosol, thus generating fewer radiolytically generated particles. Another way to reduce or control the intensity of the soft x-rays is to control emission of the cathode in the soft x-ray emitter. The reduced intensity of the soft x-rays was found by experiment to satisfactorily condition an aerosol stream without substantial radiolytic generation of particles precipitation.

RELATED APPLICATIONS

This Application claims the benefit of U.S. Patent Application No.61/070,880, filed Mar. 26, 2008, which is hereby incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

Aerosol neutralizers are utilized in a variety of aerosol applicationand test devices, including characterization of aerosols that aresparsely populated (e.g. the monitoring of clean room environments) aswell as aerosols that are particle laden (e.g. combustion engineexhaust, coating sprays). The primary task of the aerosol neutralizer isto condition the aerosol to obtain a reproducible, steady-statepopulation of particles having a distribution of charged (positive andnegative) particles and neutral particles that is known to within anacceptable uncertainty, and to produce such a characteristic in theaerosol regardless of the charge condition of the aerosol entering theaerosol neutralizer. By conditioning the aerosol to a known steady-statepopulation, the total concentration of particles can be inferred bymeasuring only a portion of the particle distribution (e.g., particlesof a certain size, mobility, and/or charge).

For example, some detectors detect only positively charged particleswithin a given size range. Because the steady state population ofparticles is reasonably known relative to the positively chargedparticles within the size range, the total concentration of theparticles can be inferred. An unconditioned aerosol stream may notpossess the steady state characteristics, thus rendering the inferencemeaningless.

One way to condition an aerosol is to bombard it with x-rays. The x-raysinteract with the gas in the aerosol, producing a bi-polar population ofions. These ions then interact with the particles of the aerosol,thereby transferring charge to the particles. Particles having a highcharge upon entering the aerosol neutralizer will attract oppositelycharge ions generated in the gas, thus tending to neutralize theparticle. By this same mechanism, most of the smaller particles will notsustain multiple charges of a given polarity. Larger particles cansustain multiple charges by virtue of their size. See Wiedensohler, “AnApproximation of the Bipolar Charge Distribution for Particles in theSubmicron Size Range,” J. Aerosol Sci., vol. 19, no. 3, pp. 387-389,1988. Accordingly, the steady state distribution of the aerosolpopulation comprising a mixture of neutral particles, single-chargedparticles and multiple-charged particles is rapidly attained under x-raybombardment.

Conventional neutralizing devices utilizing radioactive substances suchas americium (²⁴¹Am), krypton (⁸⁵Kr), polonium (²¹⁰Po) and the like areknown to produce a bipolar population of charged particles in anaerosol. Such devices carry with them concerns stemming from thehazardous radiation attendant the radioactive substance and from thegradual decrease of effectiveness characterized by the half-life.Americum has a half-life of 432 years and krypton a half-life of 11years, thus posing safety concerns both in terms of personnel utilizingand storing the device, and in terms of disposal of the unit when itsoperational life is at an end. Polonium has a substantially shorter halflife (138 days), which may mitigate against long term disposal concerns,but presents additional handling concerns as the radioactive substancetypically requires replacement during the operational life of theneutralizer.

Recently, U.S. Patent Application Publication No. 2006/0108537 disclosedan aerosol particle charging device that utilizes “soft” x-rays, thatis, x-rays having a wavelength in the range of approximately 0.13- to2-nm and having photon energies in the range of approximately 600- to10,000-electron-volts (eV). Soft x-ray devices eliminate concernsregarding handling of radioactive materials because the soft x-rayemitter does not utilize or produce radioactive materials nor does itemit any radiation when no power is supplied to it.

However, both the conventional and the soft x-ray devices are known togenerate particles by a process of radiolytic precipitation. Radiolyticprecipitation is the result of a cascade of events beginning withionization of individual molecules which form reactive species from gasconstituents and impurities in an aerosol, subsequently interacting witheach other to condense into particles. Hence, aerosol neutralizersutilizing x-ray devices typically add particles to an aerosol stream viaradiolytically produced particle. The generated particles are generallyundesirable, as they typically add particles of unknown size andcomposition to an aerosol, thereby distorting the aerosol that is beingcharacterized.

A neutralizer that provides the radioactivity-free operation of the softx-ray emitter without significant radiolytic generation of particleswould be welcome.

SUMMARY OF THE INVENTION

Various embodiments of the invention reduce the radiolytic generation ofparticles to a negligible amount while still fully conditioning anaerosol stream, even at high flow rates. The reduction in the radiolyticgeneration of particles is accomplished by reducing by several factorsthe intensity of the soft x-rays that bombard the aerosol flow beingconditioned. We have found that the reduction in radiolyticallygenerated particles drops exponentially in relation to the decrease inthe intensity of the soft x-rays, while the reduction in the productionof the bi-polar population in ions is only modestly reduced. The resultis that the radiolytic generation of particles is disproportionatelydiminished relative to the reduction in the generation the of bi-polarpopulation of ions. Furthermore, we have discovered that in someinstances the intensity of a standard soft x-ray emitter can be reducedby as much as a factor of 33 times and still produce a bi-polarpopulation of ions sufficient to neutralize an aerosol.

Structurally, a soft x-ray emitter is configured to irradiate an aerosolconditioning chamber through which an aerosol passes. An attenuatingwindow may be positioned between the soft x-ray emitter and the interiorpassage of the aerosol conditioning chamber so that the soft x-rayspassing therebetween pass through the attenuating window. Through properselection of material and thickness, the intensity of the soft x-raysthat irradiate the aerosol flow can strike a balance whereby theradiolytic generation of particles is insignificant for a givenapplication while the conditioning of the aerosol flow is stillcomplete.

In one embodiment, an aerosol conditioning device comprises an aerosolconditioning chamber having an inlet and an outlet and defining aninterior flow passage. A soft x-ray emitter is operatively coupled withthe aerosol conditioning chamber, with an attenuating window disposedbetween the soft x-ray emitter and the interior flow passage. Theattenuating window may be adapted to reduce the intensity of soft x-raysemitted by the soft x-ray emitter so that radiolytically generatedparticles produced by the soft x-rays within the interior flow passageis insignificant. The attenuating window may be configured to reduce theintensity of soft x-rays emitted by the soft x-ray emitter by a factorof approximately 33. The soft x-rays entering the aerosol conditioningchamber may be tailored to produce an x-ray intensity of approximately0.045 Sievert/hour or less, depending on the sensitivity of thedownstream measurements to radiolytically generated particles.

In another embodiment, a method for neutralizing an aerosol whilegenerating an insignificant amount of radiolytically generated particlescomprises providing a soft x-ray emitter operatively coupled with anaerosol conditioning chamber defining an interior flow passage, the softx-ray emitter being configured to produce soft x-rays of a ratedintensity. The x-ray emitter is then caused to emit soft x-rays. Theintensity of the soft x-rays produced by the soft x-ray emitter isreduced to provide soft x-rays of a reduced intensity relative to therated intensity. At least a portion of the reduced intensity soft x-raysis caused to enter the aerosol conditioning chamber and to pass throughthe conditioning chamber. The aerosol is thereby bombarded with the softx-rays of reduced intensity as the aerosol passes through theconditioning chamber to generate an insignificant number ofradiolytically generated particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a trend graph of radiolytic generation of particles andbi-polar population of ions vs. x-ray intensity;

FIG. 2 is a cut-away view of a soft x-ray neutralizer in an embodimentof the invention;

FIG. 3 is a block diagram of a particle generation test setup;

FIG. 4 is a graph of the particle production test for an unattenuatedsoft x-ray neutralizer at a flow rate of 1.5 liters per minute;

FIG. 5 is a graph of the particle production test for an unattenuatedsoft x-ray neutralizer at a flow rate of 0.3 liters per minute;

FIGS. 6 and 6A are graphs of the results from the particle productiontest for an attenuated soft x-ray neutralizer at a flow rate of 0.3liters per minute;

FIG. 7 is a block diagram of a conditioning test setup; and

FIG. 8 is a normalized particle distribution of the results of theconditioning test.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, a dual ordinate graph 20 having a radiolyticallygenerated particle ordinate 22 and a bi-polar population of ionsordinate 24 vs. an x-ray intensity abscissa 26 is presented. Aradiolytic production characteristic 28 and a bi-polar populationcharacteristic 30 as generated by soft x-rays are depicted on the dualordinate graph 20.

The radiolytic production characteristic 28 is generally proportional tothe x-ray intensity abscissa 26 raised to a power M, i.e.RGP∝X^(M)  Eqn. (1)where RGP is the radiolytically generated production rate, X is thex-ray intensity, and M is a value greater than unity and believed to betypically greater than 2.

The bi-polar population characteristic 30 may range from approximately asquare root function to approximately a linear function with respect tothe x-ray intensity abscissa 26:BPP∝X^(N)  Eqn. (2)where BPP is the bi-polar population at x-ray intensity X and N is inthe range of approximately ½ to 1.

A full power x-ray intensity level 34 for a typical soft x-ray emitteris depicted on the x-ray intensity abscissa 26, at which level theradiolytic production characteristic 28 is at a first value RGP1 and thebi-polar population characteristic 30 is at a first value BPP1.

Typically, BPP1 is substantially greater than is required to adequatelycondition an aerosol flow through a neutralizer. Moreover, because thepower M of the radiolytic production characteristic 28 typically greaterthan two and the bi-polar population characteristic 30 is approximatelylinear or sub-linear, the value of the RGP will decrease in greaterproportion than will the value of the BPP as the x-ray intensity X isdecreased.

Accordingly, an adequate x-ray intensity level 36 may be establishedthat is less than the full power x-ray intensity level 34, where thebi-polar population characteristic 30 is at a second value BPP2. Acorresponding value RGP2 of the radiolytic production characteristic 28is also established at the adequate x-ray intensity level 36. The dualordinate graph 20 illustrates that the proportionate change between RGP1and RGP2 is substantially greater than the proportionate change betweenBPP1 and BPP2. Therefore, while the bi-polar population of ions 24remains adequate, the radiolytically generated particle production maybecome insignificant or marginal in terms of the contribution ofparticles to the aerosol being conditioned.

Referring to FIG. 2, a soft x-ray neutralizer (SXRN) 40 is depicted inan embodiment of the invention. The soft x-ray neutralizer 40 includes asoft x-ray emitter 42 operatively coupled with an aerosol conditioningchamber 44 that defines an interior flow passage 45. An unconditionedaerosol 46 may be introduced into the interior flow passage 45 through afirst port 48. Soft x-rays 50 are emitted from the soft x-ray emitter 42and directed into the interior flow passage 45 of the aerosolconditioning chamber 44. A conditioned aerosol 54 emerges from theaerosol conditioning chamber 44 via a second port 56. An obstruction 58such as a sphere or perforated plate may be placed upstream of thesecond port 56. An attenuating window 60 may be placed between the softx-ray emitter 42 and the interior of the aerosol conditioning chamber44.

Alternatively, the flow of aerosol may be reversed. That is, theunconditioned aerosol 46 may enter the second port 56 and theconditioned aerosol 54 exit via port 48. Because the bi-polar populationof ions tends to be more concentrated near the soft x-ray emitter, thereversed flow configuration may change the residence time of thebi-polar population of ions within the aerosol conditioning chamber 44.The change in residence time of the ions can be a factor in theradiolytic generation of particles.

In operation, the unconditioned aerosol 46 may be bombarded with softx-rays 50 emitted from the soft x-ray emitter 42 as the aerosol passesthrough the aerosol conditioning chamber 44. The soft x-rays 50 interactwith the carrier gas of the aerosol to generate ions, which in turn caninteract with the particles in the aerosol to transfer charges to theparticles. The attenuating window 60 may form a fluid flow barrierbetween the internal components of the soft x-ray emitter 42 and theaerosol conditioning chamber 44.

The obstruction 58 blocks the direct line-of-sight between theattenuating window 60 and the second port 56 for enhanced safety ofpersonnel in the area. Alternatively, the blocking function may also beaccomplished by imposing a bend or turn or serpentine in the structurethat defines the second port 56.

The attenuating window 60 may be comprised of a material and thicknessthat substantially reduces the intensity of the soft x-rays 50 thatenter the aerosol conditioning chamber 44. The attenuation may betailored so that the intensity of the soft x-rays 50 is adequate tocondition the unconditioned aerosol 46 as it flows through the aerosolconditioning chamber 44, as described in the discussion of FIG. 1. Thereduced intensity of the soft x-rays 50 also provides the attendant anddisproportionately larger reduction in the production of radiolyticallygenerated particles.

Experiments were devised and executed to determine the feasibility ofthe foregoing method and apparatus. The experiments and results aredescribed below.

Referring to FIG. 3, a particle generation test setup 70 was devised totest whether the soft x-ray neutralizer 40, either attenuated orunattentuated, produced radiolytically generated particles. For thetests under discussion, the “unattenuated” configuration of the softx-ray neutralizer comprised a window of mylar of 0.004-in. thickness. Inthe “attenuated” configuration, the window 60 further included multiplelayers of aluminum foil totaling 0.005-in. thickness. (The attenuationprovided is a function of the total thickness of material, not thenumber of layers.) The attenuated configuration was found to reduce theintensity of the soft x-rays entering the aerosol neutralizing chamber44 by a factor of approximately thirty-three (i.e. from approximately1.5 Sievert/hour to approximately 0.045 Sievert/hour). The intensity ofthe soft x-rays entering the neutralizing chamber 44 was estimated byinterpolating the specifications of the soft x-ray emitter 42. The softx-ray emitter 42 used in the particle generation test setup 70 was aHamamatsu L9490, which specifies an x-ray dose at a distance of onemeter from the output window of the device of 0.015 Sievert/hour. Themidpoint of the neutralizing chamber 44 was approximately 10-cm from theoutput window. Because radiation intensity varies by the inverse of thedistance squared, the dose at 10-cm will be approximately 100 timesgreater than at 1-m, putting the dose at about 15 Sievert/hour withoutattenuation. For an attenuation factor of 33, the attenuated x-rayintensity for the particle generation test setup 70 was thereforeapproximately 0.045 Sievert/hour.

The SI unit of radiation dose is the Gray, which is the amount ofradiation that deposits 1 Joule in a kilogram of absorbing material. TheSievert is the Gray dosage multiplied by a factor that includes damageto biological tissue. For example, penetrating ionizing radiation (e.g.,gamma and beta radiation) have a factor of about 1, so 1Sievert=approximately 1 Gray. Alpha rays have a factor of 20, so 1 Grayof alpha radiation dosage=20 Sievert.

The soft x-ray neutralizer 40 was configured so that an unconditionedatmospheric flow stream 72 was drawn through a pair of high efficiencyparticulate air (HEPA) filters 74 connected in series in order to removesubstantially all particles from the flow stream and produce a clean airflow 75 that enters the soft x-ray neutralizer 40. An exit flow 76 fromthe soft x-ray neutralizer 40 was directed into a condensation particlecounter 78 (TSI model 3025) and processing software 79 (TSI AerosolInstrument Manager Software for CPC). The soft x-ray neutralizer 40 wasoperatively coupled to an on/off control 80. It is further noted thatthe soft x-ray neutralizer 40 was configured for reverse flow in theparticle generation test setup 70 (aerosol entering port 56 and exitingfirst port 48).

In operation, the soft x-ray neutralizer 40 could either be in aninactive state (i.e. controller 80 off) or an active state (i.e.controller 80 on). With the soft x-ray neutralizer 40 in the inactivestate, the condensation particle counter 78 measures only the particlesthat pass thorough the pair of HEPA filters 74. With the soft x-rayneutralizer in the active state, the particles detected by thecondensation particle counter 78 would be the combination of what passedthrough the HEPA filters 74 and the particles radiolytically generatedby the soft x-ray neutralizer 40 in operation. Because of the extensivefiltration, it was expected that the amount of particles detected by thecondensation particle counter 78 would be very low with the soft x-rayneutralizer 40 in the inactive state, and that the particles detected bythe condensation particle counter 78 would comprise substantially onlythe particles produced radiolytically by the soft x-ray neutralizer 40in the active state.

Referring to FIGS. 4 through 6, the results of the particle generationtest are presented. A high flow rate result 90 is presented in FIG. 4,wherein the particle generation test setup 70 was adjusted to draw 1.5liters per minute (LPM) of air therethrough. During this test, the softx-ray neutralizer was active and in an unattenuated configuration (thatis, with the mylar window of 0.004-in. thickness between the x-rayemitter 42 and the aerosol conditioning chamber 44). The graph of FIG. 4plots an elapsed time 92 of the experiment vs. the overall particleconcentration 94 indicated by the condensation particle counter 78 on alogarithmic axis. An interval of inactivity 96 is also presented in FIG.4, wherein the soft x-ray neutralizer 40 was in an inactive state.

The high flow rate results 90 indicate overall particle concentrationsdetectable by the condensation particle counter to be largely in therange of 0.01- to 1.0-particles per cubic centimeters (#/cc). Nodetectable particles were found during the interval of inactivity 96,implying that the particles detected during the high flow rateexperiment are purely radiolytically generated particles. Generally, aradiolytically generated particle contribution of 1.0#/cc or less isconsidered satisfactory in many applications.

A low flow rate result 100 is presented in FIG. 5, wherein the particlegeneration test setup 70 was adjusted to draw 0.3 liters per minute(LPM) of air therethrough. As with the high flow rate result 90, the lowflow rate result 100 was generated using an unattenuated soft x-rayneutralizer 40, and is characterized by a period of inactivity.

The low flow rate results 100 indicate overall particle concentrationsdetectable by the condensation particle counter to be initially in therange of 200 to 2000#/cc, and dropping to approximately 10#/cc after theperiod of inactivity 96. The detectable level of particles found duringthe interval of inactivity 96 was on the order of 0.1#/cc, which is twoto four orders of magnitude less than the concentration measurementsacquired in the active state; hence, the contribution of non-radiolyticparticles can be presumed negligible. The substantially higher particleconcentrations for the low flow rate results 100 may be problematic inmany applications.

Second low flow rate results 110 and 120 are presented in FIGS. 6 and6A, wherein the soft x-ray neutralizer 40 was arranged for theattenuated configuration. The low flow rate of 0.3 LPM was chosenbecause of the potentially problematic result demonstrated with the lowflow rate result 100.

The initial second low flow rate result 110 presents data that wasacquired over a period of approximately 2 hours. Initially, theproduction of radiolytically generated particles was approximately2#/cc, rapidly decreasing to approximately 0.1#/cc, then steadilydecreasing into approximately 0.01#/cc. Accordingly, the second low flowrate results 110 may be characterized as demonstrating that theattenuated configuration produces radiolytically generated particleconcentrations that are typically less than 1#/cc, which constitutes adecrease in the production of radiolytically generated particles of overthree orders of magnitude in some instances over the non-attenuatedcounterpart of the low flow rate results 100.

After the initial second flow rate result, additional results 120 wereobtained for approximately one more hour. During this time, nodetectable levels of radiolytically generated particles were detected.At approximately the 3400-second mark of this test, the two HEPA filters74 were removed from the system, which caused an increase 122 in thedetected particles.

Because the particle production decayed to an undetectable level, it isbelieved that the particles detected for the second flow rate result 110were the result of the attenuated soft x-rays reacting with residueand/or contaminants left behind on the exposed surfaces of the aerosolconditioning chamber 44 during previous experiments. After the aerosolconditioning chamber 44 was effectively cleansed of these effects, theattenuated soft x-rays no longer generated a detectable level ofparticles. Accordingly, the detected particles of the second flow rateresult 110 are not believed to have been the result of an interactionbetween the attenuated soft x-rays and the clean air flow 75.

Furthermore, it is noted that the particle increase at the end of thesecond flow rate result 120 is believed to have nothing to do withradiolytically generated particles. Rather, these are particles thatentered the condensation particle counter 78 from ambient by virtue ofthe attenuated soft x-ray neutralizer 40 being unfiltered. The purposeof the removal of the filters 74 was to verify that the condensationparticle counter 78 was still operating.

Accordingly, the attenuated configuration reduced the production ofradiolytically generated particles associated with the low flow ratecondition to an acceptable or insignificant level, whereas theconcentration of radiolytically generated particles produced by theunattenuated configuration at the low flow rate was significant andgenerally unacceptable.

The descriptors “high flow rate” and “low flow rate” describes only theflow rates as they relate to each other, and are not intended toindicate or imply a limitation of the invention. Also, the level ofradiolytically generated particles deemed “insignificant” or“acceptable” depends on the application. For example, in a Class 1 cleanroom environment used in semiconductor manufacture, the production ofradiolytically generated particles may need to be less than 10⁻⁴particles/cc, whereas in the monitoring of urban atmospheric aerosols aproduction of radiolytically generated particles of 1 particles/cc maybe satisfactory.

Having demonstrated that the attenuated configuration substantiallyreduces the production of radiolytically generated particles over theunattenuated configuration, a remaining question is whether theattenuated SXRN is effective for the task of conditioning the aerosol. Atest was devised and executed to determine the effectiveness of theattenuated configuration, described below.

Referring to FIG. 7, a conditioning test setup 170 is depicted to testthe attenuated configuration of the soft x-ray neutralizer 40 of theinvention. The conditioning test setup 170 included a salt aerosolgenerator 172, a diluter 174, a particle charger 176, the soft x-rayneutralizer 40 in the attenuated configuration, and a scanning mobilityparticle sizer 177 comprising a differential mobility analyzer (DMA) 178and the condensation particle counter 78, all in serial fluidcommunication with each other as depicted in FIG. 7. A vent 180 waslocated between the particle charger 176 and the soft x-ray neutralizer40. Also, a flow controller 182 was operatively coupled between the softx-ray neutralizer 40 and the scanning mobility particle sizer 177.

The diluter 174 included a first flow path 184 and a second flow path186, the flow paths 184 and 186 being in parallel with each other, thesecond flow path 186 including a high efficiency particulate air (HEPA)filter 192. The diluter 174 further included a pair of valves 194, onefor each of the flow paths 184 and 186.

The particle charger 176 was a unipolar corona-jet charger obtained froma TSI Model 3070A electrical aerosol detector, and included an on/offcontrol 196. The particle charger 176 was set up to accept a regulatedclean air flow 198 for the corona air flow. The clean air flow 198 waspassed through the particle charger 176 at a rate of approximately 0.4LPM for this work.

In operation, the salt aerosol generator 172 produced 3.5 liter/minuteof aerosol 200 that passed through the diluter 174 and comprisingparticles of salt. The valves 194 on the diluter could be adjusted toroute more or less aerosol through the HEPA filter 192, thus reducing orincreasing, respectively, the concentration of the aerosol 200 beingpassed on for testing. The net flow through the soft x-ray neutralizer40 was controlled as the sum of the fixed flow through the scanningmobility particle sizer 177 and the adjustable flow through the flowcontroller 182. The vent 180 provided a path for escape of excess testaerosol flow from the charger 176.

The particle charger 176 could either be in an active state (i.e.controller 196 on) or an inactive state (i.e. controller 196 off). Whenin the active state, the particle charger 176 imposes a charge on theaerosol 200 as it enters the attenuated soft x-ray neutralizer 40. Whenin the inactive state, the particle charger 176 is a pass-through devicethat does not substantially alter the charge distribution of the aerosol200 as it enters the attenuated soft x-ray neutralizer 40.

The purpose of the DMA 178 was to provide a variable filter that passesparticles of a given mobility determined by the voltage setting of theDMA 178. In operation this voltage is scanned while the CPC 78 measuresthe concentration of particles passing through the DMA 178. The resultis a mobility distribution measurement (often interpreted as a sizedistribution measurement).

Two tests were conducted with the conditioning test setup 170, both withthe flow rate through the attenuated soft x-ray neutralizer 40 being setat 3.0 LPM by drawing 1.5 LPM through the scanning mobility particlesizer 177 and an additional 1.5 LPM through the flow controller 182. Thehigher flow rate was chosen because the shortened residence time in theaerosol conditioning chamber 44 subjects the aerosol to less bombardmentof soft x-rays, thus posing a greater challenge to produce a conditionedexit flow.

The first test was conducted with the particle charger 176 in an activestate. The second test was conducted with the particle charger 176 in aninactive state. Accordingly, the aerosol entering the attenuated softx-ray analyzer 40 would be highly charged when the particle charger 176was activated, and would be representative of an ambient or naturalcharge when the particle charger 176 was in an inactive state.

Referring to FIG. 8, a particle distribution 220 is presented includingthe results from the active state test 222 and the results from theinactive state test 224. The particle distribution 220 presents anormalized number concentration of an aerosol 226 (dN/dlogDp, in unitsof counts/cc per log diameter range) vs. the mobility diameter 228 ofthe particles within the aerosol.

The dN/dlogDp parameter (hereinafter “normalized number concentration”)comprises the number of particles or droplets contained within amobility diameter interval normalized against the size or “bin width” ofthe interval. The normalization enables comparison of distributionshaving different bin widths. To count the total number of particleswithin a size range, one adds the normalized number concentrationswithin the size range and multiplies it by the dlogDp (bin width). Here,“dlogDp” or “bin width” is defined as the difference between the base-10logarithm of the upper limit of the interval and the base-10 logarithmof the lower limit of the interval (i.e. the “bin width” is the base-10logarithm of the ratio of the upper limit to the lower limit of theinterval).

The software 79 of the scanning mobility particle sizer 177 isprogrammed with certain assumptions in inferring the concentration ofparticles within a given bin width from a measured particle count. Theassumptions include a Fuchs charge distribution for the particles. Whenthe particle charger 176 is in the active state, the actual chargedistribution of the particles entering the attenuated soft x-rayneutralizer 40 violates this assumption. Therefore, if the attenuatedsoft x-ray neutralizer 40 is not conditioning the charged aerosolsufficiently to attain the Fuchs condition, a substantial differencebetween the two tests 222 and 224 would be observed.

Instead, the results from the two tests 222 and 224 are in closeagreement with each other. Such a result implies that the attenuatedsoft x-ray neutralizer 40 is adequately conditioning the aerosol,regardless of its charged state upon entering the neutralizer 40.

Another way to reduce the intensity of the x-rays entering the aerosolconditioning chamber 44 is to manipulate the operating conditions of thex-ray tube. The x-ray energy or wavelength spectrum is determined by thecathode-to-target voltage, together with the material of the target. Thex-ray intensity is established by the current of electrons bombardingthe target. The current of electrons is determined by the cathodeemission which may be controlled either by changing the cathodetemperature (e.g. changing the voltage applied to the cathode or thecathode heater), or through the use of additional control electrodes orgrids within the tube. Such control of the cathode emission may providean alternative or additional way to reduce the intensity of the x-raysfrom the soft x-ray emitter 42.

It is noted that x-ray tubes have a limited lifetime, and one of thefailure mechanisms is growth of crystals resulting in failure of theheated filament in the tube. Operation at full filament temperaturetends to inhibit or slow the crystallization process. The accelerationvoltage must typically be maintained in order to get the required x-rayspectrum, and with the full acceleration voltage applied and thefilament at full temperature, operation at full current and powerresults. Thus, while it is possible to reduce the x-ray intensity byreducing the filament temperature, this approach is generally notfavored because it reduces tube life. The use of control electrodes maybe preferable to reducing filament temperature, but it may not bepossible without redesign of the x-ray tube itself.

Each of the additional figures and methods disclosed herein may be usedseparately, or in conjunction with other features and methods, toprovide improved devices, systems and methods for making and using thesame. Therefore, combinations of features and methods disclosed hereinmay not be necessary to practice the invention in its broadest sense andare instead disclosed merely to particularly describe representativeembodiments of the invention.

For purposes of interpreting the claims for the present invention, it isexpressly intended that the provisions of Section 112, sixth paragraphof 35 U.S.C. are not to be invoked unless the specific terms “means for”or “step for” are recited in the subject claim.

1. A method for configuring a device to neutralize an aerosol whilegenerating an insignificant amount of radiolytically generatedparticles, comprising: providing a soft x-ray emitter operativelycoupled with an aerosol conditioning chamber defining an interior flowpassage, said soft x-ray emitter being configured to produce soft x-raysof a rated intensity; causing said x-ray emitter to emit soft x-rays;reducing the intensity of at least a portion said soft x-rays producedby said soft x-ray emitter to provide soft x-rays of a reduced intensityrelative to said rated intensity; causing at least a portion of saidsoft x-rays of said reduced intensity to enter said aerosol conditioningchamber; causing said aerosol to pass through said conditioning chamber;bombarding said aerosol with said soft x-rays of said reduced intensityas said aerosol passes through said conditioning chamber; determiningthat said soft x-rays of said reduced intensity adequately conditionsaid aerosol; and determining that said soft x-rays of said reducedintensity do not produce radiolytically generated particles above apredetermined amount.
 2. The method of claim 1, wherein said step ofproviding includes providing an attenuating window disposed between saidsoft x-ray emitter and said interior flow passage of said aerosolconditioning chamber, said attenuating window reducing said at leastsaid portion of soft x-rays to said reduced intensity in said step ofreducing.
 3. The method of claim 2 wherein said attenuating window insaid step of providing causes said reduced intensity to be approximately33 times less than said rated intensity.
 4. The method of claim 2wherein said reduced intensity is approximately 0.045 Sievert/hour orless.
 5. The method of claim 1 wherein said insignificant number ofradiolytically generated particles is approximately 1 particles/cc orless.
 6. The method of claim 5 wherein said insignificant number ofradiolytically generated particles is approximately 10⁻⁴ articles/cc orless.
 7. An aerosol conditioning device, comprising: an aerosolconditioning chamber having an inlet and an outlet and defining aninterior flow passage; a soft x-ray emitter operatively coupled withsaid aerosol conditioning chamber; and an attenuating window disposedbetween said soft x-ray emitter and said interior flow passage, saidattenuating window being adapted to reduce the intensity of soft x-raysemitted by said soft x-ray emitter and passing through said attenuatingwindow so that radiolytically generated particles produced by said softx-rays within an aerosol passing through said aerosol conditioningchamber are below a predetermined amount, said aerosol being adequatelyconditioned by said soft x-rays of reduced intensity.
 8. The aerosolconditioning device of claim 7 wherein said attenuating window isconfigured to reduce the intensity of soft x-rays emitted by said softx-ray emitter by a factor of approximately
 33. 9. The aerosolconditioning device of claim 7 wherein said soft x-rays entering saidaerosol conditioning chamber produce an x-ray intensity of approximately0.045 Sievert/hour or less.
 10. The aerosol conditioning device of claim1 wherein said predetermined amount is a particle concentration.
 11. Theaerosol conditioning device of claim 7 wherein said predetermined amountis a particle concentration.
 12. A method for determining the populationof particles in an aerosol while generating no more than an acceptableamount of radiolytically generated particles within said population ofparticles in said aerosol, comprising: providing a soft x-ray emitteroperatively coupled with an aerosol conditioning chamber that defines aninterior flow passage, said soft x-ray emitter being configured toproduce soft x-rays of a rated intensity; establishing a measurablecriterion of a desired conditioned aerosol, said conditioned aerosolhaving a substantially steady state charge distribution, said measurablecriterion relating to a sample portion of said population of particleswithin said conditioned aerosol, said measurable criterion of saiddesired conditioned aerosol being proportional to X^(N), where X is anintensity of exposure to soft x-rays and N is a power; establishing ameasurable criterion of said radiolytically generated particles anddetermining an acceptable level of radiolytically generated particleswithin said population of particles in said aerosol, said measurablecriterion of said radiolytically generated particles being proportionalto X^(M), where M is a power greater than unity and is greater than N;introducing said aerosol into said interior flow passage of said aerosolconditioning chamber; operating said x-ray emitter to emit soft x-rays;causing at least a portion of said soft x-rays emitted by said x-rayemitter to enter said aerosol conditioning chamber at a level ofintensity that provides a conditioned aerosol having a criterion withinsaid measurable criterion of said desired conditioned aerosol andprovides said acceptable level of radiolytically generated particles;and inferring a population of particles of said aerosol from themeasurement of said sample portion of particles in said conditionedaerosol.
 13. The method of claim 12 wherein N is not less than ½. 14.The method of claim 12 wherein N is not greater than unity.
 15. Themethod of claim 12 wherein said measurable criterion of said desiredconditioned aerosol is indicative of a bipolar populationcharacteristic.
 16. The method of claim 12 wherein said level ofintensity of said soft x-rays entering said aerosol conditioning chamberin said step of causing is established by attenuation of said softx-rays.
 17. The method of claim 16 wherein said attenuation of said softx-rays causes said level of intensity of said soft x-rays to beattenuated by a factor of approximately 33 or greater.
 18. The method ofclaim 16 wherein said level of intensity of said soft x-rays enteringsaid aerosol conditioning chamber is approximately 0.045 Sievert/hour orless.
 19. The method of claim 12 wherein said acceptable level ofradiolytically generated particles is approximately 1 particles/cc orless.
 20. The method of claim 19 wherein said acceptable level ofradiolytically generated particles is approximately 10⁻⁴ particles/cc orless.